Construction material based on activated fly ash

ABSTRACT

The disclosure concerns cost effective concrete formulations based on an alkali activated binder. The construction material of the concrete type, contains sand, fine aggregates, coarse aggregates, water and a binder comprising:
         from 55 to 80 wt. % of fly ash containing less than wt. 8% of CaO;   from 15 to 40 wt. % of blast furnace slag;   a chemical activator containing:
           from 0.8 to 4 wt. % of alkaline silicates; and   from 1.5 to 9 wt. % of alkaline carbonates;   wherein the chemical activator has an silica to alkali molar ratio from 0.1 to 0.55; and   
           a booster comprising at least one strong base.       

     The disclosure also concerns a method to produce such a concrete construction material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Entry of International ApplicationNo. PCT/IB2007/002377, filed on Aug. 17, 2007, which is incorporated byreference herein.

TECHNICAL FIELD

The present invention concerns concrete formulations based on an alkaliactivated binder with no cement or clinker additions (mixtures of flyashes and slags) that provides strength development and workabilitysimilar to ordinary Portland cement based concretes.

BACKGROUND OF THE INVENTION

Fly ash is a by-product of burning coal, typically generated during theproduction of electricity at coal-fired power plants. Fly ashes aremainly composed by aluminosilicates partially vitrified, as well asmineral phases such as quartz, hematite, maghemite, anhydrite and so onwhich had been present as impurities in the original coal. ASTM C 618-85(“Standard specification for fly ash and raw calcinated natural pozzolanfor use as a mineral admixture in Portland cement concrete”) hasclassified fly ash into two classes, Class C and Class F, depending onthe total sum of silica, alumina and ferric oxide present. Class Fcontains more than 70% of the above oxides and Class C contains lessthan 70% but more than 50%. Class F fly ash is typically low in calciumoxide (<8%) whereas Class C has a higher content being sub-classified intwo categories: Class Cl (8-20% CaO) and Class CH (>20% CaO). Therefore,Class F fly ash is not usually considered as a cementitious material byitself because, due to its low calcium oxide content, it cannot beagglomerated after hydration to produce bonding strength in the finalproduct, contrary to Class C fly ash.

Fly ash is a by-product that has to be used and consumed to reduce itsenvironmental impact. Nowadays, it has mainly been used as a partialsubstitute in ordinary Portland cement due to its pozzolanic reactivity.However, there is a limitation in the replaced quantity because thepozzolanic reaction rate is very low at room temperature causing initiallow strength and fast neutralization.

Recently trials have been carried out to increase the pozzolanicreaction rate by using activators such as alkaline and alkaline earthcompounds (ROH, R(OH)₂), salts from weak acids (R₂CO₃, R₂S, RF) andsilicic salts type R₂O(n)SiO₂, where R is an alkaline ion from Na, K orLi. However, either the activation efficiency is not enough or there aresome undesired interactions between ordinary Portland cement andactivators, which causes rheological and/or mechanical problems. Thisfact promotes the use of additional components, mainly admixtures, whichincreases the complexity of the formulation and makes worse thetechnological development of these products.

The high amount of lime CaO in fly ash type C provides the waste productwith intrinsic cementitious properties. On the other hand, fly ash typeF does not by itself develop any strength on hydration, and anactivation of the product is requested to ensure that strengthdevelopment will take place on hydration. A major advantage to preferfly ash type F rather than fly ash type C is the high availability inlarge quantities of fly ash type F and its lower market price. Sincetransportation costs of industrial wastes would be a key issue for thecost effectiveness of the final product or binder, the selection of flyash type F is guided by its availability in large quantities and itsdense geographic distribution.

For many years, many formulations and processes have been proposed toactivate fly ash or industrial wastes in order to use it as acementitious material. U.S. Pat. Nos. 5,435,843 and 5,565,028 describedthe activation of Class C fly ash at room temperature with strong alkali(pH>14.69) to yield cementitious properties. Even though there is noexpress mention of Class F fly ash use in these patents, the cementcontaining Class C fly ash according to these patents has limitedapplication due to the corrosive properties (pH>14.6) of the usedactivators.

EPO Patent No. 0858978 discloses that high volumes of activated Class CFly ash (>90%) may be used as a cementitious binder. The binder containsa mix of Class C and Class F Fly ashes wherein the dosage of Class F flyash has to be limited up to 60% due to its low reactivity. In this case,Class F Fly ash is mentioned but it is used together with clinker andadmixtures like citric acid, borax, Boric acid, which are veryexpensive, and KOH, which is corrosive (pH>13). Furthermore,formulations get complex because the high number (>6) of presentedcomponents.

In a similar way, U.S. Pat. No. 5,482,549 describes a cement mixturecontaining at least 2% by weight of Portland cement clinker, 2-12% byweight of sodium silicate, fly ash and blast furnace slag. The patentspecifies that the fly ash has to be ground to a specific surface ofmore than 500 square meters per kg which is very important and yieldshigh manufacturing costs (energy consumption, handling, etc.).Furthermore, this document doesn't mention the use of Class F fly ash.

Xu et al., “The activation of Class C-, Class F-Fly Ash and BlastFurnace Slag Using Geopolymerisation”, 8th CANMET/ACI InternationalConference on Fly Ash, Silica Fume, Slag and natural Pozzolans inConcrete, Las Vegas, Calif., USA (2004), shows that Class F fly ash canonly be properly activated when using a highly alkaline soluble silicatesolution. Following this line, U.S. Pat. No. 5,601,643 proposes aninvention related with chemically-activated fly ash cementitiousmaterials, preferably Class F Fly ash, where high content of alkalimetal and/or alkaline earth metal silicate are used to obtain highstrength cementitious mixtures. However, this invention has a limitedapplication because: 1) a high curing temperature is need, 2) a high pH(>14, corrosive products) is required and therefore, safety conditionsare necessary to handle the mixture and 3) the cost of the mixture ishigh due to the high quantities of soluble silicates and alkalis used.Furthermore, formulations related with high alkalis content and high pHcause alkali-leaching problems and efflorescence due to the overdose ofactivators. The overdose of activators is due to Class F fly ash that isconsidered as a binder and not as active filler, which requires lessalkaline dosage for being activated.

Skvara et al. (Ceramics Slicaty 43-1999) described alkali activatedmixtures of slag and fly ash using sodium silicates and sodiumhydroxides at high dosages (SiO₂/Na₂O located from 0.6 to 1.6) onpastes. PCT patent publication no. WO 2005/09770 discloses alkaliactivated mixtures of slag and fly ash in the form of pastes usinggypsum additions in the anhydrite form and superplasticizers to achievesignificant strength development. Most if not all of these studiespresent results based on pastes (sometimes on standard mortars) withlimited or no industrial interest and most of the time very high costs.

The activation of the various latent pozzolanic materials (e.g. slag,clay, fly ashes, flues, natural pozzolan) is described using varioussources of alkalis salts (silicates, carbonates, hydroxides) but most ofthe time, the respective amounts of the alkali sources is not detailed.However, experience has shown, that the source of the alkali foractivation plays an important role and that any combination does notprovide the same results. Finally, optimizing the quantity and thesource of the chemical activator is highly relevant in order to controlthe cost of the final concrete product. Furthermore, most of thepublications emphasize that curing of these pastes should occur atelevated temperatures (above 40° C.) or need a preliminary heattreatment for some hours at temperatures located between 60° C. and 100°C. Considering real industrial construction material poses otherproblems than trying to activate latent hydraulic material to developsome strength in pastes.

For concrete applications, using conventional amounts of sand andaggregates, the problem is different since workability and strengthdevelopment mechanisms are clearly affected by the aggregates and theindustrial mixing conditions. Therefore, the invention intends todescribe new industrial construction materials, mainly concrete mixdesigns that can be used in many structural applications (ready mix,pre-cast). The invention consists in providing an alternative toconventional Portland cement based concrete. Furthermore, the contentand the nature of the chemical activators have to be optimized in orderto enable effective strength development, cost effective final concreteand to avoid leaching and lixiviation problems related to unreactedexcess chemical activators.

The aim of the invention is to remedy to the above drawback by providingindustrially applicable concrete compositions with the followingadvantages:

-   -   environmental friendly;    -   easy to formulate involving limited number of components;    -   safe and easy to handle and to prepare with conventional        equipment;    -   cost effective;    -   controlled workability without additions of organic        superplasticizers;    -   ability to be prepared on the construction site; and    -   no specific curing conditions.        Typically, the invention doesn't aim to use any cement or cement        related compounds (like cement kiln dusts for instance). The        advantage not to use cement in the formulation of the binder is        mainly based on the objective of simplicity and polyvalence of        the invention. Cement or the like additions in the formulation        will lead to additional problems of interactions with the        chemical activators that need to circumvent by further specific        chemicals etc, special curing conditions, etc. The objective of        early strength development, as well as the universal property of        the binder will be very difficult to achieve. Finally, the        ecological advantages of the product according to the invention        will be reduced since cement, clinker or cement kiln dust        additions are correlated to additional CO₂ emissions. It will be        seen in the following description that none of the prior art        present the technical features and none of the prior art have        all advantages provided by the present invention.

SUMMARY OF THE INVENTION

A construction material is based on a simple formulation that is easy toproduce at room temperature and to operate, following a robustnessprocess, with similar or better properties (rheology, mechanicalstrength, durability etc.) than ordinary Portland cement (OPC), andcovering a wide scope of applications in various fields, preferably forready-mix concrete. Therefore, the aim of the invention is to provide alow cost and simple multipurpose industrial construction material,namely a range of concrete mix designs, including, sand, fine and coarseaggregates, material made from activated residues: high volumes of ClassF fly ash (>50%), small quantities of Blast Furnace Slag (<40%) and verysmall quantities of industrially available alkaline carbonate (R₂CO₃),alkaline silicate R₂O (n)SiO₂, and a booster, typically any strong basethat is used to set and control the pH. A further aim of the inventionis to provide an industrial construction material, more specificallyconcrete mix designs using conventional aggregates, that developsstrength over time (e.g. after 2, 7 or 28 days) in a similar way thatconcrete based on ordinary Portland cement (EN classes C30, C40, C50,etc.) at conventional curing temperatures (22° C.+/−2° C.). A furtheraim of the invention is to provide a set of concrete mix designs, whichfabrication costs are optimized, and complying with the industrialrequirements of a standard concrete based on ordinary Portland cement.

An important advantage of the invention is that it provides a veryrobust product and process, which is not sensitive over chemicalcomposition variations of the industrial wastes (fly ash type F andblast furnace slag). Thus, the expected mechanical resistancerequirements are always achieved. Another advantage of the invention isthat the new construction material is provided with a good workability(measured using the ASTM C143 Abraham's cone test) at low water binderratio to enable a good strength development.

Ecological advantages are present with this invention because it is afriendly cementitious binder made from residues, with limited indirectCO₂ emissions and with low energy consumption during its production.Furthermore, the alkali leachability is controlled by correct dosage ofactivators insuring their combination in the hydration products.Furthermore, the water to binder ratio and other optimized parameters ofthe concrete have been selected in order to provide good workability(e.g. Abrahams cone larger than 5 cm or larger than 15 cm to obtain socalled super fluid concretes) together with an acceptable strengthdevelopment under conventional curing temperature (room temperature).

Moreover, the various chemical components that enter the chemicalactivator have to be designed carefully to optimized the cost of theactivator (largest cost contributor to the cubic meter of concrete).Finally, the binder content of the concrete and the water to binderratio are also very important parameters to control the cost of thecubic meter of produced concrete. Other advantages will appear in thefollowing detailed description, where the invention will be betterunderstood based on exemplary embodiments and comparative examples bymeans of the following tables and figures.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows typical chemical composition of respectively class F flyash and blast furnace slag used to make the binder according to thepresent invention;

FIG. 2 shows some representative typical properties of respectivelyclass F fly ash and blast furnace slag used to make the binder accordingto the present invention;

FIG. 3 shows the mineralogical composition obtained by X-Ray diffractionof the Class F Fly ash and blast furnace slag used to make the bindercomposition according to the present invention;

FIG. 4 shows some examples of concrete mix designs using conventionalaggregates sieve line. Compressive strength on EN cubes (15 cm×15 cm×15cm) are given at 2, 7 and 28 days after curing at 22°+/−2° C.; and

FIGS. 5 and 6 show a typical aggregates sieve line curve used for theconcrete testing.

DETAILED DESCRIPTION

The binder according to the invention is manufactured from fly ash,blast furnace slag and chemical activators. The dry hydraulic binder(including the fly ash, the slag and all chemical activators) accordingto the invention comprises:

-   -   fly ash containing less than 8% w/w of CaO: 55-80% by weight;    -   blast furnace slag: 15%-40% by weight; and        a chemical activator containing:    -   from 0.8 to 4 wt. % of alkaline silicates expressed as SiO₂; and    -   from 2 to 9 wt. % of alkaline carbonates expressed as R2O.        The chemical activator is a mix of alkali silicate and alkali        carbonates (Na, K or Li). Advantageously, the fly ash is pure        class F fly ash.

Fly ash is the main component of the binder. Fly ashes are mainlycomposed by aluminosilicates partially vitrified, as well as mineralphases such as quartz, hematite, magnetite, anhydrite and so on whichhad been present as impurities in the original coal. Class F fly ashcontains more than 70% of silica, alumina and ferric oxide and typicallyless than 8% of calcium oxide.

Table 1 presents the typical chemical composition, obtained by X-Rayfluorescence, of class F fly ash (FAF) used in the scope of thisinvention. Table 1 shows that the fly ash composition matches therequirements of a type F fly ash, with a CaO content of less than 8%.The chemical composition presented in the table of FIG. 1 onlyrepresents an example and the present invention is not limited to thechemical composition of FIG. 1. Class F fly ash is mainly a finelydispersed material, with a specific surface of 250 to 350 square meterper kg. In order to limit the energy consumption and reduce themanufacturing price of the binder, the fly ash type F does not requireany pre-milling process. The table of FIG. 2 also presents some othertypical but not limiting properties of the ground BFS used in the frameof the invention.

FIG. 3 presents a typical X-Ray diffraction pattern performed on a FAF.It can be observed that the product is partially crystalline. Theprincipal mineral phases found in the FAF are described with thefollowing nomenclature: M-Mullite (alumino-silicates) O-Quartz andMG-Maghemite (iron oxide). On FIG. 3 it can also be observed the factthat the bulk material FAF also contains one or more amorphous (glassy)phases, which represents 50-60% by weight. The glassy phases,characterized by the halo presented in the background of FIG. 3 (≅19-32sin 2Θ), are mainly constituted of aluminosilicates.

The second component of the binder is blast furnace flag (BFS). BFS is ahighly impure calcium alumina-silicate glass that is a by-product fromthe pig iron production. BFS is typically used in the cement industry asa pozzolanic material in addition to Portland clinker and in theconcrete industry as an addition to the cement portion.

FIG. 1 presents the typical chemical composition of the BFS used in thescope of this invention. The chemical composition is classicallyobtained by X-Ray fluorescence. The chemical composition presented inFIG. 1 only consists in an example and the present invention is notlimited to the chemical composition of FIG. 1. FIG. 2 also presents someother typical but not limiting properties of the ground BFS used in theframe of the invention.

FIG. 3 shows a typical X-Ray diffraction pattern performed on a BFS. Itcan be observed that the product is mainly amorphous. The amorphouscontent of the BFS is typically higher than 90% in weight. FIG. 2 alsopresents some other typical but not limiting properties of the groundBFS used in the frame of the invention.

The BFS comes in the form of a granulated medium with a very lowspecific surface. The size of the individual particles varies from somemillimeters to some centimeters. Therefore, the BFS has to be groundusing a conventional industrial mill (bar mill, ball mill) in order toobtain a specific surface from 350 to 550 square meters per kilogram.This operation is very conventional in the cement and concrete industry.Thus, unlike some other binders of the prior art, the binder used inthis invention according to the present invention does not requirespecial grinding or milling operation to increase the specific surfaceto very high values (over 650 square meters per Kg).

It has to be stated here that the milling energy varies exponentiallywith the fineness. Thus, requirements for high specific surface yieldenormous production costs in energy and in milling capacity of theindustrial mills since the duration of the milling has to be drastic toreach elevated fineness. The present invention does not require specificBFS grinding operations and complies with the values that areconventional for cement industry (350 to 550 square meters per Kg). As aconsequence, the invention makes it possible to use ground BFS from anormal milling terminal, yielding no additional costs. Typically, twodifferent finenesses of slag were used characterized by a specificsurface of respectively around 400 square meters per Kg (Blaine 4000)and around 500 square meters per Kg (Blaine 5000).

The chemical activator is designed to provide the main source of alkalisfor the alkali activation reaction with the fly ash and the slag. Unlikeother binders of the prior art, the chemical activator contains 2 mainsources of alkalis: alkaline silicates and alkaline carbonates. In orderto meet the objective of cost reduction, Sodium will be the preferredselected alkali according to a first embodiment, but it is clear thatlithium and/or potassium can advantageously replace or partiallysubstitutes the sodium for some applications. The respective dosage ofthese alkalis sources is performed to optimize the costs of theactivator, to enable targeted strength development and workability ofthe final concrete as will be shown in the examples.

The sodium carbonate, also called soda ash and the sodium silicate arecommercially available in large industrial quantities and exist in solidform (powders) and in liquid form. Alkaline carbonates and silicic saltstype, R₂O (n)SiO₂, called alkaline silicate, where R is an alkaline ionfrom Na, K or Li are the activators. Advantageously, the silica toalkali molar ratio of the activator is located between 0.1 and 0.55.Although alkali silicates present the advantage to provide a veryconcentrated alkali source, the selection of the alkali carbonates asactivator is motivated by economical reasons since carbonates are cheap,widely available in the form of powdered material.

The chemical activator is aimed to provide the optimized quantity ofalkalis and silicates to initiate the reaction with the blast furnaceslag and the fly ash to form hydrated gels and later on to enable theinorganic polymerization to develop strength. Excess alkalis orsilicates will not take place in the formation of the hydrated products(gels and inorganic polymer) and will remain unbounded, leading toleaching and lixiviation problems in the concrete. The range ofsilicates to alkalis ratio, as well as the activated content, accordingto the invention enables to optimize the strength development avoidingsilicates or alkalis leaching issues in the final concrete.

For costs and industrial availability reasons, the carbonates used aremainly sodium and potassium carbonates, although Lithium may also beconsidered. The ratio between sodium and potassium carbonates is one ofthe parameters that permits to further optimize the early age strength(e.g. strength at 2 days) and a good workability of the concrete mix atgiven water to binder ratio. Advantageously, the molar ratio of Na₂O toK₂O for the carbonates is located between 0.6 and 7, preferably between1.7 and 3.5.

The water used for the invention does not require any particularprecaution and it can be considered that any water that would be usedadvantageously for an Ordinary Portland Cement (OPC) can be use withoutrestriction with the binder according to the present invention. Thewater is added to the construction material in a water to binder ratiolocated between 0.3 and 0.45. Thus, the water content is sufficient toprovide good workability (Abrahams cone values above 5 cm) of theconstruction material and permits a strength development underconventional curing temperature.

Preferably, all the components of the chemical activators will bediluted into part or all of the water requested to meet the selectedwater binder ratio to prepare the concrete. Dissolving the chemicalactivator into the water will typically yield a pH value located between11 and 12 that is insufficient to fully initiate the reaction in case ofconcrete application and would not yield industrially acceptable earlyage strength (e.g. around 10 MPa at 2 days). Furthermore, such a pHvalue does enable to provide robust industrial concrete mix designs.

Therefore, a booster comprising a least a strong base is used, in smallquantity, to set the pH to values typically located between 12.1 and13.7, preferably between 12.5 and 13.5, depending on the desiredproperties (strength development and workability). The pH value is setand measured taking into account the overall quantity of water requiredby the selected water binder ratio of the concrete mix design. Thebooster comprises a strong base or any strong bases mixes. The strongbase may be an organic or an inorganic base and may be chosen among:e.g. hydroxides of Li, Mg, K, Na, Ba, Cs Ca, Sr or organic strong baselike butyl lithium or sodium amine. Typically, depending on the desiredpH, the design of the chemical activator and the water to binder ratio,the strong base (booster) concentration in the total water comprised inthe construction material is located between 0.05 and 2.5 molar.Although the preferred method is to add all the chemical activators andthe strong base booster in the entire water needed for the mix design,it is clear that only part of the needed water may contain the chemicalactivators and the booster whereas some complementary plain wateradditions may be performed to finally obtain the required amount ofwater of the mix.

Here, it can be seen that the number of components used in theformulation of the binder according to the invention is very limited andindustrially available at low costs. Furthermore, none of the componentsrequire any specific pre-treatment and can be used from the conventionalmanufacturing processes without yielding additional costs.

The aggregates used in the concrete testing are typically described onFIG. 5 and it can be seen that a conventional design of the aggregatessize distribution has been used (Fuller-type). Tests were performed onround aggregates (3 fractions with respectively sand 42.7% volume,gravel 4-8 mm 22.3% volume and gravel 8-16 mm 35% volume) and crushedaggregates using for instance 9 fractions to meet the same sieve line(granulometry distribution) as described in FIG. 5. No difference inflow and strength development could be noticed using round (3 fractions)or crushed aggregates (9 fractions) for a given mix design.

In order to provide a construction material that would have the sameflexibility and the wide range of applications of an ordinary Portlandcement (OPC)-based concrete, it is important not only to focus theattention on the strength that will develop after 28 days but also toconsider the strength at the early stage. The resistance after 2 days isin that respect an important value for many applications (pre-cast,slabs, building construction, etc.). Typically, an EN standard concretebased on an ordinary Portland cement (with a content of 350-450 kg percubic meters of concrete), would yield resistance strength incompression to values from 10-30 MPa after 48 hours at room temperature.

It is one goal of the invention to achieve similar early strengthwithout having to use special curing conditions at elevated temperature(vapor curing, etc.) in order to respect the polyvalence, theflexibility and the low cost of the binder. Unlike other bindersdescribed in the prior art, the binder according to the invention doesnot require any special curing to enable acceptable strength developmentafter 48 hours. It will be shown that the compressive resistanceobtained after 2 days using standard curing conditions is identical tothe compressive resistance of an ordinary Portland cement in the sameconditions.

Of course, the invention is not limited to the described components. Onecan for instance consider alternatives involving for instance theaddition of the components by industrial and agricultural residuescontaining high alkalis or highly reactive amorphous silica. Forexample, silica fume, rice husk ashes or natural aluminosilicates likevolcanic pozzolanes or zeolites can also be added to the binder. We willnow describe some applications and mixes. EN concrete test samples(cubes 15 cm×15 cm×15 cm) were prepared using a conventional concretemixer with aggregates and sand typically described on FIGS. 5 and 6although it is clear that the invention is not limited to this specificsand and aggregates granulometry. The sand, the aggregates the slag andthe fly ash were dry mixed in the concrete mixer and then the watercontaining the dissolved activator and the strong base booster wereadded for further mixing.

Various parameters were tested to analyze the flow (Abraham's cone) andthe strength development of the mix designs:

-   -   the content of the binder (slag+fly ash+chemical activator) from        350 Kg to 600 Kg per cubic meter;    -   the water to binder ratio from 0.3 to 0.5;    -   the pH of the water containing the dissolved activator and the        base booster from 12.5 to 13.7;    -   the design of the chemical activator with SiO2/R2O from 0.1 to        0.55;    -   the design of the chemical activator containing from 0.8-4%        weight of alkaline silicates (SiO2) and from 1.5 to 9% weight        alkaline carbonates (R2O);    -   the slag content from 15 weight % to 40 weight % of the total        binder; and    -   the fly ash content from 55% in weight to 80% in weight of the        total binder.        Also as mentioned 2 different finenesses were used for the slag,        typically around 350-400 square meter per Kg and around 500        square meter per Kg.

FIG. 4 shows typical examples of the strength development of concreteaccording to the invention at 2, 7, and 28 days using compressive test.Preparation, curing and testing were performed at room temperature innormalized conditions. The strength development is in accordance to theexpectation and mortars present a very good early strength at 2 days andan improved very high resistance at 28 days with respect to ordinaryPortland cement.

Controlling and setting the pH with help of the strong base booster isvery important to maintain high workability at low water to binder ratioand therefore providing good strength development. For instance, givenconcrete mixes design (constant constituents and water/binder ratio)will exhibit higher workability when the pH is increased from 12.5 to13.0 and to 13.5 for instance. According to a second aspect of theinvention, we will now describe an optimized method to prepare theconcrete according to the invention for concrete, ready mix and pre-castapplications.

The following process enables to further improve the properties of theconcrete. According to the invention, the method to produce a concreteconstruction material comprises the following steps:

-   -   a) In a first mixer, preparing the hardening slurry mixture by        homogenisation of the chemical activator, the blast furnace        slag, and part or all of the required water;    -   b) In a second mixer, preparing an initial dry mix by        homogenisation of sand and aggregates (fine and coarse) and        class F fly ash; and    -   c) Conventional mixing the initial mix b) with the hardening        mixture slurry a) and adding the rest water if needed.        The booster may be added during steps a) or b) but is preferably        added during step a).

According to a preferred embodiment of the invention, the step ofhardening slurry mixture preparation comprises:

-   -   a first step of preparing an activator solution by dissolution        of the chemical activator and the booster in all or part of the        required water; and    -   a second step of mixing the activator solution with the blast        furnace slag.

Thus, the method is safer because heat release will occur only duringthis activator solution preparation step and will not occur duringfurther step. Furthermore, large quantities of activator solution can beprepared in advance. In both cases described hereabove, the strong basebooster additions to set the pH of the overall water+chemical activatorat a given value between 12.1 and 13.7 are typically but not necessarilyadded to the activator solution. Adding the booster to activatorsolution will be favourable to enable the pH measurement usingconventional industrial equipment. If the booster is not added to theoverall required water, calculations need to be done to take intoaccount the quantity of additional water that has to be added to reachthe desired water to binder ratio. Thus, the final mixture can beachieved efficiently since the hardening mixture obtained under a) isvery fluid.

According to this method, the overall mixing duration does not exceedthe mixing duration of a normal process. Preferably, all the requiredwater will be used during step a) that enables an easier control of thepH. Nevertheless, the process according to the invention includes usingpartially the required water to dissolve the chemical activator.Finally, a wide range of organic and inorganic admixtures can be addedto the formulation if necessary (in a similar way to standard concretebased on OPC) to modify the properties of the concrete (air entrainer,superplastizers, retarders, accelerators, etc.).

1. A construction material, containing sand, fine aggregates, coarseaggregates, water and a binder, the construction material comprising:(a) from 55 to 80 wt. % of fly ash containing less than wt. 8% of CaO;(b) from 15 to 40 wt. % of blast furnace slag; (c) a chemical activatorcomprising: from 0.8 to 4 wt. % of alkaline silicates; and from 1.5 to 9wt. % of alkaline carbonates; and wherein the chemical activator has ansilica to alkali molar ratio from 0.1 to 0.55; and (d) a boostercomprising at least one strong base.
 2. A construction materialaccording to claim 1 wherein the binder is mixed with water in a waterto binder ratio located between 0.3 and 0.45.
 3. A construction materialaccording to claim 1 wherein the booster is arranged to set the pH ofthe water to values located between 12.1 and 13.7.
 4. A constructionmaterial according to claim 1 wherein the booster comprises a least onestrong base, the molar concentration of the said strong base(s) beinglocated between 0.05 and 2.5 with regard to the water content.
 5. Aconstruction material according to claim 1 wherein the strong base(s) isa strong alkali.
 6. A construction material according to claim 1 whereinthe fly ash is pure class F fly ash.
 7. A construction materialaccording to claim 1 wherein fly ash has a specific surface from 200 to500 square meters per Kg.
 8. A construction material according to claim1 wherein blast furnace slag has a specific surface from 350 to 600square meters per Kg.
 9. A construction material according to claim 1wherein the binder content is located between 350 kg and 600 Kg percubic meter of the construction material.
 10. A construction materialaccording to claim 1 comprising industrial or agricultural residuescontaining amorphous silica.
 11. A method to produce a concreteconstruction material, containing sand, fine aggregates, coarseaggregates, water and a binder, the construction material comprising:(i) from 55 to 80 wt. % of fly ash containing less than wt. 8% of CaO;(ii) from 15 to 40 wt. % of blast furnace slag; and (iii) a chemicalactivator comprising: from 0.8 to 4 wt. % of alkaline silicates; andfrom 1.5 to 9 wt. % of alkaline carbonates, wherein the chemicalactivator has an silica to alkali molar ratio from 0.1 to 0.55; (iv) abooster comprising at least one strong base comprising the followingsteps: a) preparing an activator slurry by dissolution of the chemicalactivator, the blast furnace slag in all or part of the required water;b) preparing an initial dry mix by homogenisation of sand fine andcoarse aggregates, and class F fly ash; c) adding the activated solutiona) and the remaining water to the dry mix b) and mixing the concrete;and d) adding the booster during steps a) or b).
 12. A method to producea concrete construction material according to claim 11, wherein thebooster is added in order to reach a pH located between 12.1 and 13.7.13. A method to produce a concrete construction material according toclaim 12, further comprising a step of measuring the pH and a step ofsetting the pH with booster addition.